Lightning protection systems for wind turbine blades

ABSTRACT

A blade tip assembly for a wind turbine blade, comprising a conductive blade tip module, a receptor arrangement spaced from the conductive blade tip module, a coupler that electrically couples the conductive blade tip module to the receptor arrangement and an insulating member that insulates the coupler. The invention also can be expressed as a method for assembling a blade tip assembly for a wind turbine blade, the method comprising providing a blade tip module; providing the blade tip module with a coupler for electrically coupling the blade tip module to a down conductor of a lightning protection system; and encasing the coupler with an insulating member.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/029,886, filed Apr. 15, 2016 (pending), which is a U.S. NationalPhase Application of International Application No. PCT/DK2014/050333,filed Oct. 15, 2014 (expired), which claimed the benefit of UK PatentApplication No. 1318381.9 filed Oct. 17, 2013, the disclosures of whichare incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates wind turbine blade structures andassociated fabrication processes for improving the resilience of windturbine blades to lightning strikes.

BACKGROUND

Wind turbines are vulnerable to being struck by lightning; sometimes onthe tower, nacelle and the rotor hub, but most commonly on the blades ofthe turbine. A lightning strike event has the potential to causephysical damage to the turbine blades and also electrical damage to theinternal control systems of the wind turbine. Wind turbines are ofteninstalled in wide open spaces which makes lightning strikes a commonoccurrence. Accordingly, in recent years much effort has been made bywind turbine manufacturers to design wind turbines so that they are ableto manage effectively the energy imparted to them during a lightningstrike in order to avoid damage to the blade and the associated cost ofturbine down-time during blade replacement.

Lightning protection systems for wind turbine blades are known. In oneexample, an electrically conductive lightning receptor element isarranged on an outer surface of the blade to receive a lightning strike.Since the receptor element is electrically conductive, lightning is morelikely to attach to the receptor element in preference to the relativelynon-conductive material of the blade. The receptor element is connectedto a cable or ‘down conductor’ that extends inside the blade to the rootand from there connects via an armature arrangement to a charge transferroute in the hub, nacelle and tower to a ground potential. Such alightning protection system therefore allows lightning to be channelledfrom the blade to a ground potential safely, thereby minimising the riskof damage. However, the discrete receptor elements are relativelycomplex to install during fabrication of the blade and, moreover, theyleave a significant portion of blade area exposed to a risk of lightningstrike.

Observation of the effects of lightning strikes on turbine blades hasrevealed that the highest proportion of lightning strikes happen at theblade tips. To address this, WO2005/031158 proposes a turbine bladehaving a solid metal tip. Although the solid metal tip provides a robustconductive body to withstand a high number of lightning strikes, in somecircumstances lightning may still strike the blade in-board of the tipthereby having the potential to cause blade damage.

A further lightning protection is described in WO2013/007267, whichproposes demarcating a turbine blade into a plurality of differentzones, each zone being provided with a different protective measure thatis selected depending on the expected impact of a lightning strikewithin that blade zone. Here, a high risk strike zone such as the tipcan be protected with robust protection measures such as a solid metaltip, whilst protective measures in low risk strike zones can potentiallybe removed altogether.

It is against this context that the invention has been devised.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a blade tip assembly for awind turbine blade, comprising a conductive blade tip module, and acoupler extending from the blade tip module for attaching the blade tipmodule to a down conductor, wherein an insulating member encases thecoupler.

When installed in a wind turbine blade, the blade tip assembly isconnectable to a down conducting system so that electrical energy from alightning strike is channelled away from the tip to the lightningmanagement systems installed in the tower of the wind turbine. In knownconductive blade tips of wind turbines, it has been observed thatlightning may be attracted to the down conducting system and other metalcomponents in the blade just inboard of the blade tip which can causedamage to the blade. Beneficially, therefore, the blade tip assembly ofthe invention incorporates a receptor arrangement that is spaced fromthe blade tip module, but spaced from it, so that it serves to‘intercept’ lightning strikes that may be disinclined to attach to theblade tip such as may occur when the blade is at horizontalorientations. The receptor arrangement that forms part of the blade tipassembly therefore provides an ‘easy’ electrical grounding path whichguards against lightning attaching to metal components internal to theblade.

The receptor arrangement may include a conductive receptor base, forexample brass, that is plate like in form and which serves as anattachment point for one or more receptor elements or bolts thatpunctuate the blade skin to connect to the receptor base. In oneembodiment, the receptor is a plate that extends transverse (i.e.,chord-wise) to the longitudinal axis of the blade tip assembly

The coupler connects the blade tip module to the receptor arrangementand may be formed integral to the blade tip module. Alternatively, thecoupler may be detachable from the blade tip module. Although thecoupler could take various forms, in one embodiment it is a tongue-likeplate that extends from the blade tip module and which is encased by theinsulating member. Preferably, the insulating member is a polymericblock-like member that extends from the blade tip module and encases thecoupler and the receptor arrangement. Preferably, the insulating memberis moulded directly onto the coupler.

Although the conductive blade tip module may comprise a core memberencased in a conductive layer, a module of solid metal provides atougher unit that has a higher current-carrying capacity. In principleit is acceptable for the blade tip module to be made from any metal ormetal alloy, although currently preferred is copper.

The coupler may comprise a tongue-like attachment plate which extendsfrom the blade tip module and which may be integral with the blade tipmodule or attached to it by a suitable means, such as a set of bolts, inwhich case the coupler may be receivable in a cavity defined in theblade tip module.

In one embodiment, the insulating member is moulded directly to thecoupler and has an outer chord-wise profile similar to that of the bladewithin which it is received, in use. The relatively large size of theinsulating member ensures a strong connection between the blade tipassembly and the blade.

In a preferred embodiment, the blade tip module is solid metal, and theinsulating member may also encase a receptor arrangement within it.

The invention also extends to a method of assembling a blade tipassembly for a wind turbine blade, the method including: providing ablade tip module; providing the blade tip module with a coupler forelectrically coupling the blade tip module to a down conductor of alightning protection system; and encasing the coupler with an insulatingmember.

Advantageously, therefore, the coupler that extends from the blade tipmodule and serves to connect to a down conducting system is encased inan insulating member. The insulating member may be formed in a suitableshape, for example a shape that lends itself to incorporation into aninternal volume of a composite blade of a wind turbine. The insulatingmember therefore serves as a mounting point.

Before encasing the coupler in the insulating member, the coupler may beattached to a down conductor at a suitable junction. Therefore, duringblade assembly the blade tip assembly can be installed as a unittogether with the down conductor which makes for more convenientmanufacture.

As an alternative, a receptor base may be arranged such that it isspaced form the blade tip module and connected between the coupler andthe down conductor prior to encasing the coupler, and the receptor basein the insulating member.

It will be appreciated that preferred and/or optional features of thefirst aspect of the invention may be combined with the other aspects ofthe invention, and vice versa. The invention in its various aspects isdefined in the independent claims below and advantageous features aredefined in the dependent claims below.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, some embodiments of theinvention will now be described with reference to the followingdrawings, in which:

FIG. 1 is a plan view of a wind turbine blade equipped with a lightningprotection system;

FIG. 2 is an enlarged view of a region of the turbine blade in FIG. 1,showing in more detail apparatus relating to the lightning protectionsystem;

FIG. 3 is a section through the turbine blade in FIG. 2 along the lineA-A;

FIG. 4 is a section through the turbine blade in FIG. 2 along the lineB-B;

FIG. 5 is a section view of the leading edge of the turbine blade inFIG. 1 along the line C-C;

FIG. 6 is a view of a blade region like that in FIG. 2, but shows analternative embodiment of the lightning protection sub-system; and

FIG. 7 is a perspective view of an alternative blade tip assembly.

DETAILED DESCRIPTION

With reference to FIG. 1, a wind turbine blade 2 incorporates alightning protection system 3. The blade 2 is formed from a blade shell4 having two half-shells. The half-shells are typically moulded fromglass-fibre reinforced plastic (known as ‘GFRP’ or, simply ‘GRP’) thatcomprises glass fibre fabric embedded in a cured resin matrix. Theprecise construction of the blade shell 4 is not central to theinvention and so further detailed description is omitted for clarity.

The blade comprises a root end 6, at which the blade 2 would be attachedto a rotor hub of a wind turbine, a tip end 8, a leading edge 10 and atrailing edge 12. A first surface 14 of the blade 2 defines anaerodynamic profiled surface that extends between the leading edge 10and the trailing edge 12. The blade 2 also includes a second surfacealso extending between the leading edge 10 and trailing edge 12, whichis not shown in the plan view of FIG. 1, but which is indicated asreference numeral 16 in FIGS. 3 and 4, for example.

When the blade 2 is attached to a rotor hub of a wind turbine, airflowstrikes the surface 16 of the blade 2 and for this reason the surface 16is also referred to as a ‘pressure side’ or ‘windward side’ in the art.Conversely, the surface 14 is referred to as the ‘suction side’ or‘leeward side’.

Turning to the lightning protection system 3, this is based on a‘zoning’ concept in which the blade 2 is demarcated in a longitudinal or‘span-wise’ direction into regions or ‘zones’ depending on theprobability of receiving a lightning strike and severity of the strikein that region. A similar principle is described in WO2013/007267.

In this embodiment, the blade 2 is divided into three zones for thepurposes of lightning protection—these are illustrated in FIG. 1 aszones A, B and C. The lightning protection facility that is used in eachof the zones is selected based on a set of lightning strike parameters,such as peak current amplitude, specific energy, impulse shape and totalcharge that the blade 2 is expected to withstand in each of the zones. Abrief explanation of the different zones now follows, by way of example.

Zone A extends from the root end 6 of the blade to approximately 60% ofthe blade length in the span-wise direction. In this zone, the blade 2has a low risk of a lightning strike and so will be expected to receivea low incident of strikes and low current amplitudes, which isacceptable for blade structural impact. In this embodiment, the blade 2is not equipped with any external lightning protection within this zone.

Zone B extends from the end of zone A to approximately 90% of the bladelength in a span-wise direction. In this zone the blade 2 has a moderaterisk of lightning strike and is expected to withstand moderatelyfrequent direct lightning strike attachments having increased impulsecurrent, peak current and total charge transfer. Accordingly, the blade2 is provided with a first lightning protection sub-system 20 in theform of a surface protection layer.

Finally, zone C extends from the end of zone B to the tip end 8 of theblade 2. In this zone the blade 2 is subject to a high likelihood oflightning strikes and is expected to withstand peak current amplitudesof in excess of 200 kA and total charge transfer in excess of 300Coulombs and, moreover, a high incident of strikes. To provide therequired level of protection for the blade, zone C includes two furtherlightning protection sub-systems. Firstly, there is provided an array ofreceptors (hereinafter ‘receptor array’) 22 and, secondly, there isprovided a blade tip assembly 24. Both the receptor array 22 and theblade tip assembly 24 are electrically connected to a down conductingsystem 26, comprising first and second down conductors 28, 30 runningalong the length of the blade 2 from the tip end 8 to the root end 6,generally being arranged adjacent the leading edge 10 and trailing edge12 of the blade 2, respectively. Details of the receptor array 22, theblade tip assembly 24 and the down conducting system 26 will bedescribed later.

As has been mentioned, zone B includes a surface protection layer 20which is a conductive layer formed over the surface of, or integratedinto, both the upper half-shell and the lower half-shell of the blade 2.The conductive layer 20 may be a metallic screen or mesh, but preferablyan expanded metal foil that acts to attract lightning strikes over alarge area of the blade and which is connected to the down conductingsystem 26 in a manner that will be described. The thickness of theconductive layer 20 is such that the aerodynamic profile of the blade 2is unaffected and so it is preferred that the conductive layer isbetween 1 mm and 5 mm in thickness. The precise structure of the surfaceprotection layer 20 is not central to the invention and so will not bedescribed in further detail here.

The blade tip assembly 24 and the receptor array 22 will now bedescribed in more detail with reference to FIGS. 2 to 7. FIG. 2illustrates an enlarged portion of the blade 2 in zone C and so showsthe blade tip assembly 24 and the receptor array 22 in more detail. Theblade 2 is shown here as having the blade tip assembly 24 attached toit. However, it should be appreciated that the blade tip assembly 24 isinstallable as a unit into the blade 2 during a manufacturing process.

The blade tip assembly 24 comprises a conductive blade tip module 32 andan insulating member which, in this embodiment, is a non-conductiveinsert member 34 coupled to the blade tip module 32. The blade tipmodule 32 is preferably formed from solid metal so as to provide anextremely robust lightning receptor at the very tip of the blade 2, ableto withstand a high number of direct lightning attachments withoutsuffering damage and without requiring frequent maintenance and/orinspection. The blade tip module 32 is preferably solid copper or acopper alloy such as bronze, and is cast in such a shape so as toprovide an aerodynamic tip profile for the blade 2, and particularly thehigh curvature regions of the leading and trailing edges which areparticularly attractive to lightning strike attachment. Forming theblade tip module 32 in solid metal achieves a high melting point, whichis a major factor in avoiding damage during lightning strikes, and alsoprovides good electrical conductivity into the down conducting system.It will be appreciated that metals and alloys other than copper wouldalso be suitable.

It is preferred that the blade tip module 32 is long enough to encompassthe high curvature regions of the leading and trailing edges 10, 12 ofthe blade, although due to the solid metal tip being of relatively highmass, it is generally an aim to design the tip to be as small aspossible without reducing the ability to attract lightning to the tip.Generally, therefore, the blade tip module 32 has a length that is lessthan 1% of the total blade length and, more preferably, below 0.5% ofthe total blade length. By way of example, on a blade with a length of60 m, the tip may be approximately 10 cm in length.

The blade tip module 32 is attached to the insert member 34 by way of acoupling member or ‘coupler’ 36 shown here as an attachment plate. Theplate 36 has a rectangular shape and is secured by bolts 35 at one ofits ends to a correspondingly shaped cavity 37 provided at an attachmentface 38 of the blade tip member 32. The plate 36 is conductive and ispreferably solid metal such as stainless steel, although other metals(e.g., copper) or metal alloys are acceptable.

The plate 36 provides a means by which the insert member 34 may beattached to the blade tip module 32. In this embodiment, the insertmember 34 is injection moulded around the plate 36 which is placed intothe mould, with or without the blade tip module 32 attached to it, priorto the start of the moulding process.

The moulded insert member 34 provides a solid base by which the bladetip assembly 24 may be incorporated into a blade 2 during a fabricationprocess. For example, during the lay-up of the blade, suitable bladeskin and other structural components such as glass fabric layers orpre-preg materials may be arranged in a truncated blade mould that doesnot have a mould surface shaped to define a blade tip. The blade tipassembly 24 may then be arranged with respect to the mould such that theinsert member 34 lies adjacent the prepared blade skin components andthe blade tip module is located against the truncated end of the mould.In the conventional way, the insert member 34 may then be incorporatedinto the blade 2 during a resin impregnation process so that, followingcuring, the blade tip assembly 24 forms an integral part of the blade 2,the insert member 34 providing a bonding surface to hold the entireassembly in the blade shells.

In this embodiment the insert member 34 is polymeric, and it ispreferred that the material has a high dielectric strength, for examplein excess of 25 kV/mm which surrounds the internal metal parts with aminimum thickness of approximately 10 mm. A suitable material would bepolyurethane.

As can be appreciated from FIG. 2, the insert member 34 is comparativelywide and has a width greater than 50% of the width of the correspondingsection of the blade 2. The profile of the insert member 34 generallycorresponds to the aerodynamic profile of the blade 2.

Since the plate 36 is removably attached to the blade tip module 32,removal and replacement of the blade tip module 32 during the life ofthe blade 2 is permitted. However, the plate 36 could alternatively beformed as an integral part of the blade tip module 32 during casting.

In order to channel energy from a lightning strike on the blade tipmodule 32 to the blade root end 6, the blade tip assembly 24 isconnected to the down conducting system 26. In one embodiment, as shownin FIG. 7, a single corona-inhibiting conducting cable of the downconducting system 26 is connected directly to the plate 36 by a suitableconnecting method such as exothermic welding. A robust conductivecoupling is therefore provided between the plate 36 and the downconducting system 26. In such an arrangement, it is preferred, althoughnot essential, that the cable is welded to the plate 36 prior to theinsert member 34 being moulded over the plate 36 in which case the cableforms a unit with the blade tip assembly 24 for the purposes of theblade fabrication process.

However, in the illustrated embodiment, the down conducting system 26 isconnected to the blade tip assembly 24 in a different way. As shown inFIG. 2, the blade tip assembly 24 also includes a receptor arrangement40 which, in this embodiment, is in the form of an elongate block-likereceptor base 42 formed from a conductive material, and preferably ametallic material such as brass. The receptor base 42 extends transverse(i.e., chord-wise) to the longitudinal axis of the blade tip assembly 24and is spaced (in a span-wise direction) from the blade tip module 32 atthe far end of the insert member 34.

The receptor base 42 is electrically linked to the blade tip module 32by a conductive link 43 which, in this embodiment, is a cable that issheathed in a corona-inhibiting material, so as to suppress streamersand corona initiating from the cable 43. In contrast to the embodimentof FIG. 7, in this embodiment, both the first and second down conductors28, 30 of the down conducting system 26 are directly connected to thereceptor base 42. As before, preferably the down conductors 28, 30 arewelded to the receptor base 42 prior to formation of the insert member34 to encapsulate the components within it so that the insert member 34encapsulates the plate 36 and at least part of the receptor arrangement.It will be appreciated therefore that the plate 36, the cable 43 and thereceptor base 42 electrically couple the metal tip and the downconducting system 26. In principle, it should be noted that any numberof electrical components could form the coupler between the tip and thedown conducting system.

The receptor base 42 forms a receiving point for a set of receptorelements 44 that punctuate the shell of the blade 2 and couple to thereceptor base 42. As best shown in FIG. 3, the receptor elements 44 takethe form of bolts having a shank 44 a that extends into the blade 2 andis received into respective sockets 42 a formed in the receptor base 42,and a head 44 b that is countersunk into the shell so that an upper faceof the head 44 b sits flush with the surrounding surface of the blade 2so as not to affect the blade aerodynamics.

In order to avoid potential ‘flashovers’ between the bolt head 44 b andany nearby internal conductive components of the blade skin during alightning strike, the head 44 b is provided with an insulatingprotective collar 46, which is preferably formed from a suitableengineering plastics such as PEEK (polyetheretherketone). The protectioncollar additionally provides protection against collateral heat damageto the blade skin during a lightning strike to the receptor elements.

The insulating member 34 is sandwiched between an interior of theleeward surface 14 and an interior of the windward surface 16. Adhesive(not shown) is located between the insulating member 34 and theinteriors of leeward and windward surfaces 14, 16 to bond the insulatingmember to the interior of the blade. The insulating member 34substantially fills the hollow volume between the leeward surface 14 andthe windward surface 16 at the tip of the blade. The insulating member34 extends in a chordwise direction from the leading edge of the bladetowards the trailing edge of the blade, and in a spanwise direction fromthe conductive blade tip 8 towards the root of the blade. In thisexample the insulating member 34 is a single block which encases thecoupler 36 and the receptor base 42. The use of a single block meansthat the receptor base 42 and the coupler 36 can be provided as singlemodule which allows for easy installation into the blade.

The receptor elements 44 are mountable on the blade 2 after it has beenmanufactured with the blade tip assembly 24 in position. Followingcompletion of the blade 2, suitable apertures are created in the shellof the blade 2 and suitable tapped holes in the receptor base 42 arecreated for securing the receptor elements 44 in position. In order toguard against the loosening of the receptor elements 44, suitablelocking means (not shown) may be provided which may take the form of aplastic pellet or patch received in a lower part of the shank 44 a thatengages the receptor base 42.

In this embodiment, two receptor elements 44 are provided, both on theleeward surface 14 of the blade 2. However, it will be appreciated thatfurther receptor elements may also be provided on the leeward side ofthe blade 2 if desired.

From the above discussion, it will be understood that the blade tipassembly 24 provides two receptor points for lightning attachments:firstly the metal blade tip module 32 and, secondly, the receptorarrangement 40. The benefit of this is that a lightning strike will beattracted to the receptor elements 44 rather than attaching to the downconducting system 26 or other internal metal parts of the system. By wayof further explanation, during the high electric field environmentalconditions immediately preceding a lightning strike, the enhancedelectric field at the tip of the blade 2 will induce ionization of theatmosphere and charge transfer from the blade tip module 32 therebyunder certain conditions resulting in a ‘space charge’ surrounding thetip end 8. The existence of this space charge can in some circumstancesmodify the electric field around the blade tip thus deflecting thereturn stroke away from the conductive blade tip module 32 so thatlightning attaches to conductive components within the blade, forexample components of the down conducting system 26. The receptorarrangement 40 of the blade tip assembly 24 provides intentionalstreamer emission points adjacent to the tip and so may serve to‘intercept’ the lightning strike, the lightning strike attachingproperly to the receptor elements 44 and the receptor base 42 instead ofattaching to other less shielded components of the blade 2. This greatlyreduces the risk of blade damage at various blade angles.

Turning now to the tip receptor array 22. As has been mentioned abovethe blade tip assembly 24 is connected to the down conducting system 26which comprises first and second down conducting cables 28, 30 which runthe span-wise length of the blade 2 to its root end 6. The downconducting system 26 is also connected to the tip receptor array 22 ofthe blade 2, which will now be described in further detail.

As shown in FIG. 2, the tip receptor array 22 comprises a plurality ofreceptor components 50 that are distributed adjacent the leading andtrailing edges 10, 12 on the leeward surface 14 of the blade 2. In thisembodiment, eight receptor components 50 are provided, and each receptorcomponent 50 is coupled to a first receptor element 52 provided on theleeward surface 14 of the blade and a second receptor element 54provided on the windward surface 16 of the blade 2. Note that thewindward receptor elements 54 are not shown in FIG. 2, but are shown inFIG. 4.

With reference now to FIG. 4, which shows the structure of the receptorcomponents 50 and the receptor elements 52, 54 in more detail, each ofthe receptor components 50 includes a block-like receptor base 56 and aninsulating member 58 that encapsulates the receptor base 56.

The receptor base 56 is conductive, and preferably brass. The insulatingmember 58 is moulded directly to the receptor base 56 and so serves tosuppress the initiation of leaders from the receptor base 56 duringhighly charge environmental conditions, which thereby guards against alightning strike directly onto the receptor base 56 rather than on areceptor element 52, 54. Each receptor base 56 include a correspondingrecess 60 through which the down conductors 28, 30 are routed so as toconnect the receptor bases 56 into the down conducting system 26.Therefore, the encapsulation of the receptor bases 56 also encapsulatesthe junction between the down conducting system and the receptor bases56. The insulating members 58 are formed of a suitable polymer having ahigh dielectric strength, and it is envisaged that the insulatingmembers will be polyurethane, although other insulating materials areacceptable.

The insulating members 58 are sandwiched between an interior of theleeward surface 14 and an interior of the windward surface 16. Adhesive(not shown) is located between the insulating members 58 and theinteriors of the leeward and windward surfaces 14, 16 to bond theinsulating members to the interior of the blade. It should beappreciated that FIG. 4 shows a cross section through the insulatingmembers 58 and that the receptor bases 56 are fully encapsulated by theinsulating members 58. The insulating members 58 may have a width in thespan-wise direction of around 15 cm.

Although not shown in FIG. 4, the receptor components 50 may also beinstalled in suitable locations within the interior of the blade 2 byspacing elements that space and hold each of the receptor components 50in a predetermined position relative to the blade shell.

As mentioned, each receptor base 56 connects to two receptor elements52, 54: one on each of the leeward and windward surfaces 14, 16 of theblade 2. This arrangement reduces the number of receptor bases requiredwhich reduces the cost and weight of the blade as a whole. In addition,the receptor bases only need to be installed in one of the bladehalf-shells during blade assembly which speeds up the fabrication timeand simplifies the fabrication process. The receptor components 50 areattached to the blade half shell by adhesive, such as an epoxy adhesive.

Each receptor element 52, 54 is in the form of a bolt having a shank 61and a head 62. The shank 61 extends into the blade 2 engages into asocket 64 in the receptor base 56. The head 62 lies against and iscountersunk into the blade shell so that an upper face of the head 62 isflush with the surrounding surface of the blade 2. Like the receptorelements 44 provided on the blade tip assembly 24, the receptor elements52, 54 of the tip receptor array 22 are also provided with a polymericcollar 66 to guard against surface flashovers during a lightning strike.The protection collar additionally provides protection againstcollateral heat damage to the blade skin during a lightning strike tothe receptor.

Since the receptor components 50 are installed in region of the blade 2that has a relatively shallow depth, preferably the receptor elements52, 54 are joined to the receptor bases 56 offset from one another or‘staggered’ in a spanwise direction of the blade, as is shown in FIG. 4.This avoids the shanks 60 of opposing receptor elements 52, 54contacting one another when installed.

In the embodiment of FIG. 4, the receptor component 50 includes asingle-piece receptor base 56 which connects to two receptor elements52, 54 provided on the leeward and windward surfaces 14, 16 of the blade2. Different configurations of receptor components are envisaged, andone such example will now be described with respect to FIG. 5.

FIG. 5 is a cross-section through the blade 2 in FIG. 1 along the lineC-C, and therefore illustrates a section of the surface protection layer20 as described above.

As has been discussed, the surface protection layer 20 includes aconductive screen that provides a relatively large surface area tocapture lightning strikes, from where the energy can be channelledsafely into the down conducting system 26 without causing damage to thenon-conductive composite structure of the blade 2. Note that the surfaceprotection layer 20 is provided on both leeward and windward surfaces14, 16 of the blade 2.

In order to connect the surface protection layer 20 to the downconducting system 26, a set of receptor components 70 are provided.There are four receptor components in total, one at each corner of thesurface protection layer 20, although only one of the receptorcomponents 70 is shown in FIG. 5.

In FIG. 5, the receptor component 70 is positioned adjacent the leadingedge 10 of the blade 2. The receptor component 70 is shaped to fill moreeffectively the volume in the relatively deep profile of this region ofthe blade 2, compared with the relatively shallow profile in the regionof the tip receptor array 22.

The receptor component 70 comprises first and second receptor bases 72,74 that are encapsulated by an insulating member 76 that is generallyannular in form. More specifically, the insulating member 76 in thisembodiment is C-shaped, being defined by first and second arm portions76 a, 76 b that extend from each end of a yoke portion 76 c. Each of thereceptor bases 72, 74 is encapsulated by a respective one of the armportions 76 a, 76 b and in this manner the receptor bases 72, 74 arelocated in a predetermined position against a respective leeward 14 andwindward surface 16 of the blade 2.

A first receptor element 80 electrically couples the surface protectionlayer 20 on the windward surface 16 to the first receptor base 72. In asimilar manner to the embodiments discussed above, the receptor element80 is in the form of a bolt having a shank 80 a and a head 80 b: theshank 80 a extending through the blade 2 and engaging with the firstreceptor base 72; the head 80 b being arranged to lie flush with thesurrounding surface of the surface protection layer 20. An identicalarrangement is provided to couple the surface protection layer 20 on theleeward surface 14 to the second receptor base 74.

A conductive link 82 is provided to electrically connect the firstreceptor base 72 to the second receptor base 74 and, in this embodiment,the conductive link 82 is a zinc coated copper braided wire. Althoughbraided wire is not essential, it is useful from a manufacturingperspective since it is flexible and so can be suitably shaped to extendbetween the first and second receptor bases 72, 74 prior toencapsulation.

The insulating member 76 is sandwiched between an interior of theleeward surface 14 and an interior of the windward surface 16. Adhesive(not shown) is located between the insulating member 76 and theinteriors of the leeward and windward surfaces 14, 16 to bond theinsulating member to the interior of the blade. It should be appreciatedthat FIG. 5 shows a cross section through the insulating member 76 andthat the first receptor base 72, the second receptor base 74 and theconductive link 82 are fully encapsulated by the insulating member 76.The insulating member 76 may have a width in the span-wise direction ofaround 15 cm.

Connection is made to the down conducting system 26 by welding the firstreceptor base 72 to a corresponding down conductor, which as illustratedis the first down conductor 28 near the leading edge 10 of the blade 2.For efficient assembly, the conductive link 82 and the down conductor 28may be arranged in a predetermined pattern with respect to the first andsecond receptor bases 72, 74 and connected thereto by exothermic weldingto ensure the electrical integrity of the connection prior to castingthe insulating member 76 around the components. In this way, thereceptor component 70 becomes installable as a unit together with thedown conducting system 26. Welding is one option for connecting theelectrical components although other suitable techniques such asmechanical clamping are acceptable

In a variation of this, it is envisaged that the down conducting system26 including both the first and second down conductors 28, 30 may beinsulated as a unit with the receptor components 50 associated with thetip receptor array 22 as well as the receptor components 70 associatedwith the surface protection layer 20. FIG. 6 illustrates this, and itcan be seen that the first and second down conductors 28, 30 and thereceptor components 50 associated with the tip receptor array 22 areencapsulated by an elongate insulating element 90 that forms a unitaryinsulating housing. The insulating element 90 may take the place of theinsulating elements 58 of the receptor bases 56, although currently itis envisaged that the insulating element 90 will be a further insulatingmeasure in addition to the encapsulated receptor bases 56 and also thecorona-reducing shielding of the down conductors 28, 30. In thisembodiment, therefore, the entire electrical system within the blade 2can be fully encapsulated prior to installation and can be installed asa single unit, rather like a wiring loom, into a blade half-shell duringthe blade lay-up phase. The encapsulation of the down conducting system26 in an electrically isolating polymer reduces the probability oflightning strike attachment directly to the down conductors 28, 30 byincreasing the dielectric breakdown strength of the system

The first and second down conductors 28, 30 are themselves high voltagecorona inhibiting cable insulated cables, insulated with either a crosslinked polymer or silicone rubber. These insulated cables are thenencapsulated into the elongate insulating element 90.

The insulating element 90 can be formed from polyurethane. In aparticular embodiment, the stiffness of the polyurethane can vary alongthe length of the blade so that the down conducting system 26 has moreflexibility in some parts of the blade than others. For example, it isdesirable for the insulating element 90 to be flexible in the tip regionof the blade where the blade deflects the most under wind loads, whereastowards the root of the blade the insulating element 90 can be stiffer.

The skilled person will appreciate that variations may be made to theillustrated embodiments without departing from the inventive concept, asdescribed by the claims. Some variations to the illustrated embodimentshave been discussed above. Others will now be explained.

The invention is described in the context of a turbine blade having aso-called ‘structural shell’ construction, in which longitudinal carbonribs or pultrusions are integrated into the upper and lower shell-halvesduring the lay-up phase. However, the invention is also applicable toblades constructed according to a different design philosophy such as astructural spar construction.

The blade tip assembly 24 has been described as including a blade tipmodule 32 of solid metal. Although this is preferred for reasons ofconductivity and resilience to blade strikes, it could also be formedfrom a non-conductive core having a conductive outer layer, althoughrobustness may be adversely affected.

In the insulating arrangement described above, the insert member 34 isdescribed as single part. However, it should be appreciated that itcould also be fabricated from multiple parts.

In the above embodiments, it has been described that the blade isdivided into three zones, A, B, and C, for the purposes of lightningprotection. It will be appreciated that this is merely an example of howa blade may be configured for lightning protection and is not intendedto be limited. For example, a blade may be configured so that zone A isomitted. In effect, therefore, the blade is protected along its entirelength instead of leaving a zone relatively unprotected from lightningstrikes.

The invention claimed is:
 1. An electrical system for a wind turbineblade, comprising: a down conductor; and a blade tip assembly coupled tothe down conductor, the blade tip assembly comprising: a conductiveblade tip module; a coupler extending from the conductive blade tipmodule; a lightning receptor arrangement electrically coupled to thedown conductor and spaced from the conductive blade tip module, whereinthe coupler electrically couples the conductive blade tip module to thelightning receptor arrangement; and an insulating member that encasesthe coupler, the lightning receptor arrangement, and a portion of thedown conductor, wherein at least the coupler, lightning receptorarrangement, insulating member, and down conductor of the electricalsystem comprise an assembled, self-contained unit for placement in awind turbine blade or a wind turbine blade mould.
 2. The electricalsystem of claim 1, wherein the coupler is detachable from the blade tipmodule.
 3. The electrical system of claim 1, wherein the coupler isreceivable in a cavity defined in the conductive blade tip module. 4.The electrical system of claim 1, wherein the coupler is in the form ofan elongated plate.
 5. The electrical system of claim 1, wherein theinsulating member is moulded to the coupler.
 6. The electrical system ofclaim 1, wherein the insulating member includes an aerofoil profile in achord-wise section.
 7. The electrical system of claim 1, wherein theinsulating member is polyurethane.
 8. The electrical system of claim 1,wherein the conductive blade tip module is a solid metal tip.
 9. Theelectrical system of claim 1, wherein the lightning receptor arrangementincludes a conductive receptor base, to which a lightning receptorelement is attachable.
 10. The electrical system of claim 9, wherein theconductive receptor base is brass.
 11. The electrical system of claim 9,wherein the conductive receptor base is plate-like in form.
 12. Theelectrical system of claim 11, wherein the conductive receptor baseextends transverse to a longitudinal axis of the blade tip assembly. 13.The electrical system of claim 1, wherein the coupler includes aconductive link connected between the lightning receptor arrangement andthe conductive blade tip module.
 14. The electrical system of claim 13,wherein the conductive link is a cable.
 15. The electrical system ofclaim 1, wherein the insulating member extends between the lightningreceptor arrangement and the conductive blade tip module.
 16. Theelectrical system of claim 1, wherein the insulating member is a singleblock that encases at least part of the coupler and at least part of thelightning receptor arrangement.
 17. A wind turbine blade assemblycomprising the electrical system of claim 1, and a wind turbine bladecomprising a windward shell and a leeward shell.
 18. The wind turbineblade assembly of claim 17, wherein the insulating member is sandwichedbetween the windward shell and the leeward shell.
 19. The wind turbineblade assembly of claim 17, wherein the insulating member extends in achordwise direction from a leading edge of the blade towards a trailingedge of the blade, and in a spanwise direction from the conductive bladetip module towards a root of the blade.
 20. A method for assembling anelectrical system for a wind turbine blade, the method comprising:providing a blade tip module; providing a lightning receptor arrangementspaced from the blade tip module; and providing a down conductor;connecting the lighting receptor arrangement to the blade tip module viaa coupler; connecting the down conductor to the lightning receptorarrangement; and with the electrical system outside the wind turbineblade or wind turbine blade mould, encasing the coupler, the lightningreceptor arrangement, and a portion of the down conductor in aninsulating member so that at least the coupler, lightning receptorarrangement, insulating member, and down conductor form an assembled,self-contained unit.
 21. The method of claim 20, wherein the coupler isremovably attached to the blade tip module.
 22. An electrical system fora wind turbine blade having an interior cavity defined by a blade shell,the electrical system comprising: a down conductor; a blade tip assemblycomprising a coupler electrically connected to a receptor arrangementthat is electrically connected to the down conductor, the couplerconfigured to be connected to a conductive blade tip of the blade tipassembly; and a pre-formed insulating member configured to be receivedin the interior cavity of the wind turbine blade adjacent a tip of theblade; wherein the pre-formed insulating member encases the coupler, thereceptor arrangement and a portion of the down conductor, wherein thereceptor arrangement is configured to be spaced from the conductiveblade tip module when the conductive blade tip module is attached to thecoupler, and wherein at least the coupler, lightning receptorarrangement, insulating member, and down conductor of the electricalsystem are pre-assembled and configured as a pre-manufactured unit. 23.The electrical system of claim 22, wherein the pre-manufactured unit ispositionable within the wind turbine blade subsequent to the manufactureof the blade.
 24. The electrical system of claim 22, wherein thepre-manufactured unit is positionable within a blade mould duringmanufacture of the wind turbine blade.
 25. A method of manufacturing awind turbine blade, comprising: providing a wind turbine blade mould;positioning glass fibre layers in the mould to form a blade skin of thewind turbine blade; moulding the wind turbine blade; and coupling theelectrical system of claim 22 to the blade.
 26. The method of claim 25,wherein the coupling step occurs after moulding the wind turbine blade.27. The method of claim 25, further comprising positioning theelectrical system within the blade mould prior to moulding the windturbine blade such that the electrical system is coupled to the bladeduring manufacture of the blade.
 28. The method of claim 25, furthercomprising attaching the blade tip module to the coupler of the bladetip assembly.